In conventional vertical spindle pulverizers, coal particles or other raw materials are dropped into a rotating grinding bowl where they are crushed into fine particles by means of multiple rollers. These pulverizers are known as roll-and-race pulverizers. All of the various known pulverizers utilize crushing pressure, impact and attrition between grinding surfaces to reduce materials to a fine particle size. Air is forced upwardly around the periphery of the bowl to lift the particles and carry them away where they are combusted (e.g., in a power plant boiler). In the so-called bowl mill, the grinding rolls are stationary and the bowl rotates. The pulverized product, such as pulverized coal, is fluidized and transported via air through the pulverizer outlet and is pneumatically conveyed through pipes to a boiler, for example.
Inevitably, there are a number of ungrindable objects (e.g., rocks, shale, iron pyrites, tramp iron, etc.) which are too heavy to be fluidized by the transport air. The reject material, therefore, is pushed over the edge of the bowl and falls into the air inlet section of the mill, underneath the grinding bowl. The reject material must be removed continuously from the underbowl area to prevent excessive build-up and potential fires and/or explosions. The current device employed for removal of reject material is attached to and rotates with the bowl hub or skirt near the floor of the mill. The so-called plow or scraper is typically very heavy, difficult to maintain, and expensive. In some cases, it is also unreliable and can even damage the mill floor. The plow or scraper rotates in a horizontal plane and scrapes the rejects off of the mill floor and down an opening into a reject chute, where the rejects are collected in a hopper and then they are disposed.
There has not heretofore been provided a mill sweep device having the advantages provided by the present invention.